"uuid","repository link","title","author","contributor","publication year","abstract","subject topic","language","publication type","publisher","isbn","issn","patent","patent status","bibliographic note","access restriction","embargo date","faculty","department","research group","programme","project","coordinates"
"uuid:387ced34-9191-4914-8be4-8181f1bb1aec","http://resolver.tudelft.nl/uuid:387ced34-9191-4914-8be4-8181f1bb1aec","Tidal Bridge Dynamics: Optimising the dynamic response due to wind, waves and current","Dorgelo, Gerjan (TU Delft Civil Engineering and Geosciences)","Uijttewaal, Wim (mentor); Molenaar, Wilfred (graduation committee); Vaniushkina, Valentina (graduation committee); Reedijk, Bas (graduation committee); Delft University of Technology (degree granting institution)","2020","Currently, there is no infrastructure between the two Indonesian islands Flores and Adonara. The islands are separated by the Larantuka Strait, which has a width varying between 600 and 1000 meters. The local government would like a bridge between the two islands. However, as the water in the 18 meter deep strait is heavily subjected to tidal forces - creating a tidal amplitude of about 1.5 m and tidal flow velocities ranging up to 4.5 m/s - structural design is challenging. As traditional bridges were deemed too expensive, a new type of bridge was introduced: the `Tidal Bridge'. The pendulum founded floating bridge, which is proposed to span the deepest 400 meters of the cross-section, is designed with tidal turbines attached to the bottom of the structure. The energy production mitigates the financial burden that the 225 million US dollar Palmerah Tidal Bridge will bring. At the time of writing, a pre-feasibility study has been performed by Antea, proposing initial structural dimensions. BAM took over the design process, which lead to questions regarding the dynamic stability of the design. The objective of this report is to answer the following two research questions: 1. How can the dynamic response due to two-dimensional forcing of a Tidal Bridge be determined? 2. What design choices can further optimise the dynamic behaviour of a Tidal Bridge? A numeric tool has been created to predict the dynamics of the proposed design as function of an input of wind, waves, and current. Based on (experimental) literature, hydrodynamic coefficients determining the fluid-structure interaction were determined. The complex shape of the floaters did not allow appropriate validation of the accompanying added mass and radiation damping coefficients. Comparison with a model constructed in Ansys Aqwa showed values of similar magnitude, but precise magnitudes could not be determined. In order to find these important coefficients, a set of experiments has been performed to determine the added mass (moment of inertia) and radiation damping (moment of inertia) for heave and roll motion. The experiments showed that the added mass equations for heave were well defined, while the added mass moment of inertia equations for roll motion deviated up to 250 per cent. The acquired data was used to find better relations between the added mass (moment of inertia) and the floater dimensions. Please note, the empirical relations are based on limited data and with little mathematical background. Hence, the relations should be used with care. The radiation damping coefficients that were also extracted from the experiments showed no clear trend, but did confirm that the hand-calculations were of correct magnitude. Using the constructed model, forcing characteristics of the Tidal Bridges are investigated. In these computations, the Palmerah Tidal Bridge dimensions are used as a case study. It was noted that the extreme non-linearity of different elements of the problem (changing pendulum angle, hydrodynamic pressure field, and particle velocity/acceleration field) do not allow for linear approximation. While a linear mass-spring system predicts that the natural period is about 6.7 seconds, the maximum dynamic amplitude is found for wave periods of 9 seconds. This coincides with the largest expected waves for the Palmerah Tidal Bridge location. Reducing the natural period of the design is recommended. Additionally, research was done into the contribution of the various types of forcing, where it was found that traffic weight has negligible effect on the dynamics. Wind forces add only a few percent to the pendulum forces, but do have a significant contribution to the displacements. Furthermore, research into the impact of an approaching wave field showed that accelerations during first impact may overshoot the maximum steady state acceleration by more than 200 per cent. A parametric study on the dynamics of Tidal Bridges was performed, which did research into the the forcing combinations that lead to most amplification of the dynamics. It shows that different loading combinations are governing for accelerations in the three different degrees of freedom present in a two-dimensional system. In here, difference was found for the dynamic behaviour induced by waves from the two different wave directions, leading to two sets of three forcing combinations. This data was used to investigate the effect of three design parameters: the angle of the pendulum, the hinge location of the pendulum and the depth of the strait. Moreover, a sensitivity study is performed on a set of parameters defining the Tidal Bridge. It shows that the mass of the segment, the added mass, the floater length, the design turbine force, and the pendulum angle are most important if it comes to design optimisation. Based on the parametric study and the forcing characteristics, conclusions and recommendations are made for improvements of Tidal Bridge designs in general and more specifically: the Palmerah Tidal Bridge.","Tidal Bridge; Dynamics; Waves; Morison; Dorgelo; Turbines; Current; Experiments; Heave; Added mass; Roll; Pitch","en","master thesis","","","","","","","","","","","","Civil Engineering","",""
"uuid:3dec3876-36dc-40b4-99d3-c239072209fa","http://resolver.tudelft.nl/uuid:3dec3876-36dc-40b4-99d3-c239072209fa","Stability of randomly placed log bed protections","van den Berg, Mario (TU Delft Civil Engineering and Geosciences)","Uijttewaal, Wim (mentor); Hofland, Bas (graduation committee); Bricker, Jeremy (graduation committee); Buschman, Frans (mentor); Sieben, Arjan (graduation committee); Delft University of Technology (degree granting institution)","2019","Design criteria for the stability of rock filters on river beds (i.e. rock bed protection) are extensively researched and successfully applied in practice. The most common stability criteria are the Izbash and Shields criteria. These methods define a critical flow (Izbash) or parameter (Shields). Rijkswaterstaat (RWS) wants to explore a more sustainable bed protection by using logs. A pilot project is started where logs will be used as bed protection. It is yet unclear if the design and construction approaches for rock bed protections can be used for log bed protections. The most dominant aspect is that logs are cylindrical objects, while rocks are spherical. This means that the design criteria for rock filters might not be directly applicable to log filters. This research aims to verify if the Izbash and Shields criteria for rock can be used for logs to create functional and safe designs. To achieve this, two experiments are performed at the TU Delft faculty of Civil Engineering & Geosciences. The first experiment, done in a water filled tank, explores the settling behaviour of logs for multiple drop methods. Insight is gained in the settling velocities, horizontal spread and magnus effect (force exerted on a rotating object, e.g. the curve of a football spinning through the air) of logs settling in a water column. Results from these experiments are used in the second experiment. This experiment is done in a 14.3m long flow flume where a log filter is constructed. The log filter is constructed using the drop method that was preferred from the first experiment. The roughness, stability and porous flow of a log filter are investigated. The results are compared with what is known for rock filters. Tree branches were used as model logs. This was done to be able to correctly scale the results to prototype scale. Primary reason for this was the effect of bark on the roughness of a cylinder. This is difficult to replicate on model scale. Using branches of trees that will be used Saturating the model logs however was more time consuming than initially expected. Attempts were made to accelerate the process but they were futile. One method, using a diaphragm vacuum pump, could not be applied due to lack of resources. For any future research on the topic of tree branches as model logs it is highly recommended to use a vacuum pump to ensure that the maximum density is reached. From the first experiment it was concluded that large quantities of logs can still be used to create functional log filters. This is a positive result as this will reduce construction time on prototype scale. The method used was a funnel. This method was applied in the flume to create the log filters. By measuring the velocity profile for multiple discharges the roughness of a log filter was measured. This also resulted in an equation of the shear velocity as a function of the discharge. By increasing the discharge step by step, several mobility stages of a log filter were found. This resulted in a dataset that could be directly compared with the Izbash and Shields equations for rock. Higher critical Shields parameters were observed than for rock of the same diameter. The behaviour of a log filter differed from a rock filter in the transition from one stage to another (stable to mobile to transport to failure) did not occur slowly, but almost instantaneous. This behaviour is unwanted because it is difficult to monitor in what stage a log filter is if no changes can be observed between stages. Thus, although applying the equations for rock filters to log filters are conservative, the behaviour of log filters are more sudden and prone to escalation close to their critical thresholds. Final conclusion is that more research is required to better understand the significance of variables for the settling behaviour and stability. These are water depth, log diameter independent of density and vice versa, log orientation and log length to diameter ratio. This can be done by doing more experiments in a similar fashion where only one variable is changed at a time. For the application of the design criteria for rock to a log filter with regards to the pilot project, it is recommended to be conservative. Based on these experiments it is safe to assume that when the most unfavourable scenario is used (e.g. low log density, small log diameter, high flow velocities near the bed) a sufficient design is made, especially if the top layer(s) of the filter are placed parallel to the flow direction. Backfilling of the log filter did not increase the stability significantly in this research and is only be beneficial for scour protection.","Log; bed; protection; stability","en","master thesis","","","","","","","","2019-08-29","","","","","",""
"uuid:9656d66c-5d4b-4b7c-a84f-4500012f86c6","http://resolver.tudelft.nl/uuid:9656d66c-5d4b-4b7c-a84f-4500012f86c6","Flow over and around submerged groynes: Numerical modelling and analysis of a groyne flume experiment","Ambagts, Lindert (TU Delft Civil Engineering and Geosciences)","Uijttewaal, Wim (mentor); Mosselman, Erik (graduation committee); Labeur, Robert Jan (graduation committee); Sloff, Kees (graduation committee); Yossef, Mohamed (graduation committee); Delft University of Technology (degree granting institution)","2019","In submerged conditions groynes add resistance to a river, thereby increasing the water levels for a particular discharge. Groynes are modelled in two ways, either they are included in the bed topography when the grid resolution is sufficiently high or with a sub-grid parametrisation when the grid cell-size is larger than the geometry of the groyne. To investigate the flow patterns over and around groynes, data from a groyne flume experiment, as carried out by the BAW, is analysed. This to investigate to what extent a groyne behaves as a weir. The experiment is modelled in 2DH and in 3D to compare the performance of the sub-grid parametrisation with full 3D non-hydrostatic simulations.","groyne; groynes; modelling; sub-grid; parametrisations; weir formula; weirs; submerged; experiment; BAW; Delft3D; Finel; kribben; krib","en","master thesis","","","","","","","","","","","","Civil Engineering | Hydraulic Engineering","",""
"uuid:91a1c1bf-474b-4952-9485-c53f2fe353f0","http://resolver.tudelft.nl/uuid:91a1c1bf-474b-4952-9485-c53f2fe353f0","The Submerged Floating Tunnel: An experimental study on the hydrodynamics and kinematics of a submerged rectangular cylinder in a wave-current environment","Drost, Lisanne (TU Delft Civil Engineering and Geosciences)","Uijttewaal, Wim (mentor); Chen, Xuexue (graduation committee); Peters, Dirk Jan (graduation committee); Reinders, Kristina (graduation committee); Delft University of Technology (degree granting institution)","2019","This study presents the results of small scale flume experiments on a submerged rectangular cylinder subjected to a current, regular wave and combined wave-current environment. The objective of the study is to gain more knowledge about the hydrodynamics around and the kinematics of a submerged structure, to give a contribute to the research field of the submerged floating tunnel. For this study a rectangular cylinder with an aspect ratio (breadth-depth) of 2.5 is used. Two relative submergence depths (flume depth/model submergence) of 2.75 and 1.63 are tested. For all tests a still water depth of 0.7 m is applied. Waves resulting in very low KC numbers of <1 for regular waves and KC[1+U_c/U_m ]<2 for combined waves-current are generated. To create a combined wave-current environment, a current is created in the flume, to which waves are added by the wave generator. The water velocity is measured in front of the model. To approximate the water velocity at the model, a time/phase shift is added to the velocity signal. Linear wave theory is applied to approximate the amplitudes of the orbital velocities at the depth of the model. For the first part of the study, on the hydrodynamic forces, the cylinder is rigidly fixed in the flume. Due to the inertia dominance for low KC numbers, the relationship between the wave parameters and the hydrodynamic forces is well described by the relationship between the wave parameters and the water particle accelerations. The vertical hydrodynamic forces are found to be larger than the horizontal hydrodynamic forces. The force coefficients from this study are compared to coefficient found in previous studies. The drag coefficients for the only current tests agree well with the results from (Courchesne & Laneville, 1979), (Bearman & Trueman, 1972), (Nakaguchi, 1968) and (Venugopal, 2006). For the regular wave and combined wave-current conditions comparable results are found to those by Venugopal for a rectangular cylinder towed through a wave field (Venugopal, 2008). The drag coefficients in the present study show a similar trend in magnitude as in the study by Venugopal. However, the magnitudes have an opposite sign due to the velocity phase shift method applied in the present study. Nevertheless, the effect of this difference on the total force prediction is insignificant, because of inertia dominance. In general, the Morison equation predicts the measured horizontal force well for regular waves. Adding a current component to the waves results in a larger error between the computed Morison forces and the measured force. However, an increase in the magnitude of the added velocity does not lead to a significant increase of this error. The second part of the study focuses on the same cylinder, only not fixed but held in place by 4 tethers. For these tests a buoyancy to weight ratio of 1.5 is applied. The used tested angles between the tethers and the flume bottom are 30˚ and 70 ̊. The water depth, the wave types and model submergence depths are remained equal to the first part of the study. By comparing the kinematics found in three different configurations, a 30˚ tether angle combined with the largest submergence depth of ds=035 m are found to gives the smallest displacements and accelerations. To reduce the kinematics more, it is recommended to add vertical tethers to limit the vertical movement. In general, dynamic features are seen in the tethered model, influencing the magnitude of the kinematics. To predict the magnitude of the tether forces it is recommended to integrated these features in a structural dynamic model.","wave-current interaction; drag forces; intertia forces; rectangle; tether; submerged floating tunnel; Hydrodynamic; Morison's equation; Experimental; force coëfficients; kinematics; displacments; accelerations","en","master thesis","","","","","","","","","","","","Civil Engineering","",""
"uuid:8d9bbc27-3fe0-4b6d-bc45-4385cb2a016f","http://resolver.tudelft.nl/uuid:8d9bbc27-3fe0-4b6d-bc45-4385cb2a016f","The influence of surface screens on morphology in side channels","Oostdijk, Tom (TU Delft Civil Engineering and Geosciences)","Uijttewaal, Wim (mentor); Mosselman, Erik (mentor); Hofland, Bas (graduation committee); Visser, S.M.M. (graduation committee); Delft University of Technology (degree granting institution)","2018","Following from the programme Room for the River, side channels are created in the Netherlands. For their maintenance, alternatives to dredging vessels are needed because of nature development, such as surface screens, that use the flow itself to generate erosion.
A field experiment using surface screens was executed at the small side channel in the Welsummerwaard along the river IJssel for this study.
It turned out the screen has a positive effect by reducing the sedimentation in the channel. This effect is the largest when using a large angle of attack and a large penetration. Also, applying the screen at the bank instead of in the middle of the channel is profitable.
Recommendations are given for Rijkswaterstaat to further develop the concept, including increasing the influence of the screen, for example by working with batteries of screens at each bank. Eventually, this concept could be used for the maintenance of all side channels in the Netherlands.","Side Channel; Morphology; surface screen; Maintenance alternatives; bottom screen; sedimentation and erosion","en","master thesis","","","","","","","","","","","","Civil Engineering | Hydraulic Engineering","",""
"uuid:28298b28-8b7a-4c64-a5be-9561bc1180a5","http://resolver.tudelft.nl/uuid:28298b28-8b7a-4c64-a5be-9561bc1180a5","Porcupines for river training: A study on the near-field effect of porcupines","Nientker, Gustav (TU Delft Civil Engineering and Geosciences; TU Delft Hydraulic Engineering)","Uijttewaal, Wim (mentor); Sloff, Kees (graduation committee); Mosselman, Erik (graduation committee); Schuurman, F (graduation committee); Delft University of Technology (degree granting institution)","2018","Braiding rivers are characterised as highly dynamic, and experience annual morphological changes in plan- form. This dynamic nature of the river leads to navigational hindrance and risk of unstable bifurcation points where discharge distributions might switch. In pilot studies, porcupines have shown promising results in re- tarding the flow and cause sediment deposition near river banks to prevent bank erosion. However the aim now becomes to apply the porcupines on a much larger scale to increase the channel roughness and influ- ence discharge distributions of bifurcation points such that the flow is mainly diverted to the channel where the highest discharge is required. This way the main channel receives the largest discharge and therefore sedimentation in these channels is prevented. Currently it is not known how porcupines should be modelled in a numerical model. It is simply assumed that porcupines can be modelled similar to vegetation which is schematised as rigid cylinders with a certain density, drag coefficient and resulting representative roughness. No measurements are available to validate the assumed roughness of porcupines and if the hydro- and mor- phodynamic behaviour, represented by the model, is true.
In this thesis laboratory experiments are conducted to assess the near-field hydro- and morphodynamic ef- fect of porcupines and generate more knowledge about their behaviour. Experiments with a concrete bottom give a detailed insight in the hydrodynamic behaviour of porcupines without the interference of bedforms and morphological developments. Experiments with a sediment bottom give more insight in the morpho- logical development and flow patterns over time which clearly influenced the initial hydraulic behaviour. Experiments are conducted in a 12 metre long and 0.8 metre wide flume with a recirculating pump. The wa- ter level, discharge, density of the porcupine field and configuration of the field are systematically varied to identify the dependences on the drag and sedimentation/erosion volumes in the near-field domain of the porcupines. Additionally, general observations are performed, describing the flow structures and sedimenta- tion patterns in and around the porcupine field.
From fixed-bed experiments it is observed how the flow is retarded by the presence of porcupines. Flow is pushed around the field in both transverse and vertical direction. Behind the porcupines, longitudinal flow vectors are downward directed and the flow velocity near the bed is significantly reduced. It is observed that staggered porcupine grids help to retard the flow stronger and captures sediment behind the field in wider strokes. Non-staggered grids only work effectively in the line of porcupines. Between those lines barely any retardation is observed and therefore only narrow strokes of sedimentation are observed behind the lines of porcupines. The reduction in flow velocity behind the porcupines is similar to the velocity reductions ob- served in experiments with vegetation. The velocity retardation is gradually restored in longitudinal direction where the effect of porcupines gradually diminishes. For different experiments this deceleration of the flow has been observed and it follows a linear trend line, that by means of extrapolation can be used to quantify the effective retardation length in longitudinal direction. Water level differences over the porcupine field are observed indicating loss of energy, and pushing up the water level upstream. Porcupines can effectively in- fluence bifurcation points in braiding rivers this way. The local water level gradient over the porcupine field, combined with the velocity measurements, is used to determine the drag and representative roughness of the porcupines by using the equations of Baptist. The obtained values for the roughness are validated by sim- ulating the flume in SOBEK with the corresponding roughness coefficients. Comparing the measured water levels with the computed water levels by the model gave a relatively good fit indicating that porcupines can be schematised by the equations of Baptist with a few adjustments. The Reynolds stresses give an indication of the height of the bed shear stresses. Measurements show that the shear stress in the near-bed region behind the porcupines is lower or the same compared to the undisturbed velocity profile. Lower bed shear stresses indicate a reduction in sediment transport and is therefore an important mechanism to reduce the bedload sediment transport.
Mobile-bed experiments show clear erosion patterns inside the porcupine field due to the increased turbu- lence intensity generated by the porcupines themselves. For the experiment with low water levels and high flow velocities erosion is observed to be most severe, whereas in experiments with lower field density an
overall sedimentation is observed inside the porcupine field. Due to the erosion porcupines sink into the bed significantly reducing their effectiveness on retarding the flow. To prevent scour larger spacing between the porcupines seems beneficial. A growing sedimentation ridge behind the porcupines is observed that influ- ences the flow retardation even stronger, enhancing further sedimentation. Once maximum sedimentation height is reached, the ridge will migrate downstream as a growing sedimentation bar gradually sloping down towards its initial bed level. This length scale of the sedimentation bar is comparable with the retardation length scale. Combined with the migration rate of the sedimentation bar an estimation of the time scales can be obtained. Based on the mobile-bed experiments it is concluded that least erosion within the field is beneficial and therefore low water levels and high flow velocities should be avoided.
Although initial results show that porcupines show similar behaviour compared to vegetation and that the roughness can be estimated by using the equations of Baptist it has become clear that there are still major differences between the behaviour of vegetation and porcupines. Therefore further research is required to improve schematisations of porcupine behaviour, especially to improve the schematisation of the sediment transport around porcupines since no descriptions are yet available.
This has been done by conducting flume experiments with a simplified section of a braided river with gravel-bed similarity. At first the laboratory experiment was designed such that a stable and reproducible planform developed. A section, consisting of an upstream fixed Y-shaped confluence followed by an alluvial island surrounded by two channels and fixed outer banks, was used to systematically study the local effects of channel closure. The obtained results were analyzed and compared with the numerical studies of Ostanek Jurina (2017) and Schuurman et al. (2016).
Nine regions of bank retreat were both altogether and separately examined, but the graphs showed no definite answer of retreat rates being dependent on NDVI. Therefore, the results of the riverbank retreat analysis were categorised based on location, erosion mechanism, the slope, and vegetation classes. The areas where fluvial entrainment was the primary erosion mechanism showed a clear trend. When NDVI was smaller than 0.2, maximum bank retreat rates appeared to be 200 meters per year. When NDVI was higher than 0.2, bank retreat rates did not exceed 80 meters per year. On satellite images, vegetation was not observed in these areas, so the influence of vegetation remained questionable. In areas where mass failure caused bank retreat, no reduction in bank retreat was found. The results showed considerable scatter, although much more vegetation was present on these banks. Water level variability played a crucial role in the evaluation of the net effects of vegetation on riverbank stability. During low water, vegetation cannot provide the positive impacts, especially on steep river banks.
It is not possible to identify vegetation types from NDVI records only. NDVI also does not show which erosion mechanism takes place. This makes riverbank stability difficult to predict by using NDVI only, and therefore, NDVI does not seem to be an appropriate estimator for the additional effects of vegetation on riverbank stability. However, it can be used in combination with other remote sensing techniques to identify healthy vegetation areas and to make roughness estimations in river planform analyses.","Ayeyarwady River; Vegetation; Riverbank stability; NDVI; Bank retreat","en","master thesis","","","","","","","","","","","","","","21.984679, 96.055263"
"uuid:400f0dbe-c89b-4b5f-a2a4-37274cd9f976","http://resolver.tudelft.nl/uuid:400f0dbe-c89b-4b5f-a2a4-37274cd9f976","Stability of stones on mild slopes","Wendt, Emiel (TU Delft Civil Engineering and Geosciences; TU Delft Hydraulic Engineering)","Uijttewaal, Wim (mentor); Hofland, Bas (graduation committee); Kuiper, Coen (graduation committee); Jumelet, H.D. (graduation committee); Delft University of Technology (degree granting institution)","2017","The static stability of stones on mild slopes under wave attack is investigated in this research. The first part of the research is focused on reproducing the physical scale model tests regarding profile change of Kramer (2016) numerically with the model XBeach-G. The erosion profiles modelled with the bed-load transport formulas of Nielsen (2006) and Van Rijn (2007) in XBeach-G do not match the erosion profiles of the profile change experiments of Kramer (2016). The bed-load transport formulas of Nielsen (2006) and Van Rijn (2007) are not able to model the sediment transport in XBeach-G accurately. Furthermore, XBeach-G cannot determine the velocity and acceleration near the bed, because the model solves the flow due to currents and waves for a single layer. Therefore, it can be concluded that XBeach-G should not be used to describe static stability of stones on mild slopes under wave attack. For the application of dynamically stable structures (which is not investigated in this research), XBeach-G functions satisfactorily (Postma, 2016). For further research, a model that solves the hydrodynamics for multiple layers should be applied. In this way, the hydrodynamics near the bed can be used to describe the static stability of stones.
The aim of the second part of the research is to develop a design method that describes the static stability of stones on mild slopes under wave attack. The basis of this design method is the initiation of motion of a stone and the hydrodynamic forces that initiate this movement. The hydrodynamic forces and corresponding mobility parameters are determined with the velocity and the acceleration near the bottom. Using Bubble Image Velocimetry (BIV), the velocity and the acceleration are derived from the videos of the BIV experiments of Kramer (2016) with regular waves breaking on a slope. It is found from the results of the BIV analysis that the effective, adapted Shields parameter θ’McCall can be used to describe movements of stones on mild slopes under wave attack. This mobility parameter has been determined with the bed shear stress of McCall (2015), which added an inertia term to include the influence of accelerations. For initiation of motion of stones, it appears that the stability parameter θcr could be a value of 0.024 (in case no slope correction factor has been applied). To substantiate a design method that describes the static stability of stones on mild slopes under wave attack, the value of 0.024 could be used to define a threshold for initiation of motion of stones. More experiments need to be executed to optimize this value of the stability parameter. Moreover, a statistical value for the stability parameter could be used (like θcr,1%) to describe the static stability of stones by means of a certain number of stones that are allowed to move for a certain number of waves.","stability of stones; initiation of motion; Shields parameter; mild slopes; design method; erosion profiles; BIV analysis; XBeach-G; McCall (2015); Van der Meer (1988); Nielsen (2006); Van Rijn (2007)","en","master thesis","","","","","","","","2018-12-20","","","","","",""
"uuid:a86b4a5e-2efe-4551-9a19-4b32dd840ca1","http://resolver.tudelft.nl/uuid:a86b4a5e-2efe-4551-9a19-4b32dd840ca1","The influence of hydropower developments on the Mekong delta","Pronker, Jenny (TU Delft Civil Engineering and Geosciences; TU Delft Hydraulic Engineering)","Sloff, Kees (mentor); Uijttewaal, Wim (mentor); Mosselman, Erik (mentor); Huismans, Y (mentor); Delft University of Technology (degree granting institution)","2017","Rapid hydropower development is taking place in the Mekong river, which is expected to change the natural river regime. Because of the ecological and economical importance of the Mekong delta, the effects of these changes could have large consequences in the delta area. This research focuses on the hydrodynamical processes and assesses potential risks of changes to these processes, caused by hydropower development, could have on the ecology and socio-economy. Furthermore, a number of mitigation measures in the delta have been proposed to reduce the potential risks.","Mekong delta; hydropower; hydrodynamics; river engineering","en","master thesis","","","","","","","","","","","","","Initiative for sustainable hydropower, Mekong River Commission",""
"uuid:f79f4100-cf1f-4734-862d-7a28e3345f2b","http://resolver.tudelft.nl/uuid:f79f4100-cf1f-4734-862d-7a28e3345f2b","Implications of Fine Sediment Dynamics in Relation to Flushing Operations for Physical Habitats at the Meso-scale","Knook, Vera (TU Delft Civil Engineering and Geosciences; TU Delft Hydraulic Engineering)","Uijttewaal, Wim (mentor); Sloff, Kees (mentor); Blom, Astrid (mentor); McClain, Michael (mentor); Zolezzi, Guido (mentor); Delft University of Technology (degree granting institution)","2017","Worldwide, dam reservoirs loose over one percent of their storage capacity every year due to sedimentation. Proper management is therefore urgent. While drawdown flushing is considered an effective method to remove sediments from the reservoir, the increased sediment flux has an impact on the downstream environment. On the short term, several species suffer high mortality rates due to increased turbidity. Also on the long term the impacts might be significant. The deposition of fine sediments can change the habitat suitability and thereby jeopardize the conditions for recovery.
The aim of this research is to evaluate the effect of different flushing operation scenarios on the physical habitats at the ecologically relevant meso-scale. An idealised, depth-averaged hydro-morphodynamic model is set up in Delft3D-Flow, a modelling software package developed by Deltares. The model represents a reach of the Avisio, a river situated in the Eastern Italian Alps. Across this river, the Pezzè dam was built in 1952, trapping all incoming sediments. Every three years the dam is flushed, which gained more public attention over the last years. A specific reach was chosen as a reference case as it consists of a channel bar topography where fine sediments accumulate up to ten times more during a flushing event than in other, more channelised reaches. It is therefore considered the most affected by the flushing event. The reach has a length of roughly a kilometre and is situated 10 kilometres downstream of the dam.
The hydro-morphodynamic model assumes a non-erodible bed, as the coarsened river bed is not expected to move significantly during a flushing event. This bed has the shape of an alternate bar topography. The simulation of the flushing event is simplified as an influx of bed load transport with a fraction size of one millimetre, and a magnitude close to the equilibrium transport capacity. Deposition occurs upstream and to a higher extent downstream of the bars, where flow velocities are low and secondary flow aids the movement of sediments into these areas. Subsequently, a clean water peak is imposed to investigate its effectiveness in removing the fine sediments from the reach. The hydrograph of this peak is varied in shape, duration and magnitude to simulate different operation scenarios and natural rainfall runoff events.
By applying the Mesohabitat Evaluation Model (MEM), a habitat suitability model developed for the meso-scale, it was possible to divide the reach into classes based on flow velocity, flow depth and shear stress. Such classification highly depends on the governing discharge. The classes are mainly distinctive by the division between high and low energy classes and in this way show a high correlation to the deposition of fines. The model suggests that fine sediments remain in the system only when low energy classes are present. Although the MEM classification aims at the meso-scale, it follows a micro-scale approach and therefore undermines the advantages of assessment at the ecologically more relevant meso-scale. It is recommended to develop the MEM-procedure by accounting for neighbouring computational cells.
The MEM-procedure gives an useful indication of the spatial variety in deposition and erosion patterns. It however does not provide insight into the implications of sedimentation to the ecology, without the coupling with a biological model. Such a biological model describes the suitability of the physical habitat for a specific organism and thereby incorporates a functional goal.
As such a biological model could, due to time restrictions, not be applied in this research it was chosen to perform a micro-scale based suitability study. This illustrates the potential of morphodynamic modelling as a tool for habitat suitability modelling. Simplified preference curves were derived for spawning trout, of which the physical habitat requirements are sensitive to the deposition of fine sediments. It was found that the deposition of fine sediments hardly effects this habitat. When required nonetheless, any considered peak flow recovers a substantial amount of suitable habitat. This implies that, if a clean water peak of a sufficient magnitude follows the flushing operation, the impact on the spawning habitat, and probably any habitat, is minimal.
Whether such a clean water peak occurs, can be partly controlled by the dam operation, but also depends on the hydrology of the catchment. A ten-year hydrological time series of the Avisio river, measured upstream of the dam, shows that the catchment of this reservoir does not provide sufficient water to guarantee a clean water peak during any season. However, with the significant contribution of two tributaries that flow into the Avisio river between the dam and the reference reach, it is highly probable that a peak flow of sufficient magnitude occurs during high flow season. To ensure the benefits of a clean water peak, it is recommended to plan the flushing event at the start of the high water season, which lasts from May to July. For other rivers, it might be possible to adopt such a clean water peak as part of the flushing operation strategy, providing a higher level of control. Even though this study suggests that the implications of the flushing event to the physical habitats are minimal, and of no comparison to the potential direct impacts, this method of evaluation shows potential to assess other morphological relevant events and even long term morphological changes.
open structure that can separate the Eastern Scheldt estuary from the North Sea. In case that water
levels of +3.00 m NAP are expected, the steel gates of the barrier close and the hinterland is protected
against floods. Besides the protective function of the barrier, the location also shows potential for tidal
energy extraction.","Tidal energy; Eastern Scheldt storm surge barrier; scale model","en","student report","","","","","","","","","","","","","","51.6079694, 3.683066"